WASHINGTON — Former Blackwater security guard Matthew Murphy and now-indicted ex-colleague Paul Slough were friends, having survived the war in Iraq together, creating a bond that under different circumstances might have lasted a lifetime.

But from the witness stand in the Blackwater criminal trial this week, Murphy testified that he saw Slough fire at least two grenades into a car where a woman and her son died, two of 14 Iraqis killed on Sept. 16, 2007, in a downtown Baghdad square.

In three days of testimony, Murphy became the first Blackwater security guard to testify against his former associates in a trial expected to take months.

Slough is charged with manslaughter, attempted manslaughter and violating gun laws, as are two other defendants, Dustin Heard and Evan Liberty. A fourth defendant, Nicholas Slatten, is charged with first-degree murder in the death of the driver of the car. All four have pleaded not guilty.

The plan that day nearly seven years ago was that the Blackwater convoy of four heavily armed vehicles including Murphy, Slough and 17 other guards would come to the aid of a separate Blackwater convoy, and that both would return to the safety of the Green Zone.

The plan fell apart in the Nisoor Square traffic circle. Witnesses have described violence that began around the time a white Kia driven by the woman’s son rolled close to the convoy in which turret gunners Murphy and Slough were riding.

The Blackwater team had brought traffic to a halt, but the Kia moved forward ever so slightly toward the armored vehicles. Car bombs were a staple of insurgent attacks in Iraq in 2007 and especially in that nation’s largest city. On cross-examination, defense lawyers brought out the fear factor that Blackwater guards faced at that moment — including the clatter of AK-47 gunfire in the vicinity that Murphy conceded could have become a threat.

Murphy said Slough fired his M-4 rifle at the Kia, as well as at least two grenades. Murphy held his fire, he said, because he’d lost confidence in Slough’s judgment, noting that firing at a possible car bomb could have touched off any explosive. In addition, the picture before the two guards — a car with a woman in it — did not fit the usual car-bomber profile.

In court papers filed in March, prosecutors said evidence in the case will establish that Slatten fired the first shots that day, without justification, at the young man sitting in the driver’s seat of the Kia.

Murphy said a uniformed officer in the square gestured to him from a distance as if to say, “What is going on?” Murphy shrugged back, with his arms outstretched, and replied, “I don’t know, man.”

The episode, in which 18 other Iraqis were wounded, left Murphy furious, he testified. Raven 23 — the convoy’s call sign — had gone to the rescue of another convoy and ended up shooting unarmed people.

Blackwater leaders “were waiting for us” when the convoy pulled into the safety of the Green Zone, Murphy said. He would eventually lose his job at Blackwater, even though he hadn’t fired his gun. He said Slough told him he believed he was doing his job and was sorry if Murphy had a problem with it.

Before that day, the guards in Raven 23 got along, Murphy said. Afterwards, there was tension between those who thought the guards who used their weapons had acted wrongly and those like Slough who said they had been ensuring the safety of the convoy.

“I don’t hate any of these guys,” Murphy said. “My opinion changed because of what they did.”

A former Marine who served in Iraq, Murphy, now 37, said he ended up returning to Iraq and working for Blackwater after having grown bored with civilian life. As a Blackwater security guard he made 10 times what he had been paid as a Marine corporal, he said.